Early diagnosis of cancer allows for interventions with higher likelihood of success. Unfortunately, for many tumors, markers that allow in vivo identification and localization do not exist. In some cancers, for example, in ovarian cancer, various cancer-associated antigens such as CA125, CA602, CA130, CA72-4, CA546, CA19-9, and STN are known tumor markers. Each of these tumor markers, however, is based on the difference in expression level, including an increase or decrease in protein expression level in serum between normal individuals and epithelial ovarian cancer patients. Such proteins are usually expressed in no small amount even in normal cells and therefore exhibit low specificity for epithelial ovarian cancer. Hence, these proteins produce high false-positive and false-negative rates and as such, are incapable of being used as high accuracy tumor markers. A tumor marker that contributes to the early detection of primary cancer still remains to be obtained.